Method and apparatus for encoding and decoding information in a non-visible manner

ABSTRACT

The present invention relates to encoding and decoding of information using materials that are mildly absorbing radiation over a wide range of infrared wavelengths and substantially non-absorbing in the visible wavelengths. Examples of such encoding of information are bar codes and area markings. Information is encoded in markings on a base medium by depositing or intertexturing on the base medium a material where the surface dimensions, thickness and presence of the material contain the encoded information. The encoding utilizes a lower cost, more stable material than a material that is highly absorbing over a range of infrared wavelengths and substantially non-absorbing in the visible wavelengths. However, since the material is mildly absorbing in the infrared range, the signal obtained by reflecting or transmitting infrared radiation from the markings will be less distinct. Thus, inventive methods are needed to ensure that the encoded information can be decoded. Two different approaches are disclosed; (a) detecting the absorption spectrum and comparing to the known spectrum of the material in order to detect the presence and surface dimensions of the material and (b) utilizing techniques to improve the signal-to-noise such as restricting the range of wavelengths, matched filters and narrowing the bandwidth.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of prior application Ser. No.09/652,427, filed on Aug. 31, 2000, now U.S. Pat. No. 6,595,427 by Soniet al. and entitled METHOD AND APPARATUS FOR ENCODING AND DECODINGINFORMATION IN A NON-VISIBLE MANNER.

FIELD OF THE INVENTION

The present invention relates to encoding and decoding of informationusing materials that are capable of absorbing radiation over a widerange of infrared wavelengths and substantially non-absorbing in thevisible wavelengths. Examples of such encoding of information are barcodes and area markings. Specifically, in the present invention, theinformation is encoded by depositing a material having near infraredabsorption but no visible absorption on a common substrate. Methods andapparatus for retrieving the information use means for increasing thesignal-to-noise ratio of the signal containing the information relatedto the dimensions of the deposited material or use properties of theabsorption spectrum to deduce the dimensions of the deposited material.

BACKGROUND OF THE INVENTION

Providing authenticity and security for photo identification andproviding identifying information in consumer products are two of themost common applications of encoded markings such as bar codes. In manyimplementations, bar codes are black and white bars in which theinformation is encoded in the widths of the bars. The bar code is readwith an optical reader comprised of a source that emits radiation in arange of wavelengths, means of scanning the radiation across the barcode and a detector that receives the reflected radiation. Theinformation can be decoded from the electrical signal produced by thedetector since the reflectance from the black bars is significantlydifferent than that from the white bars.

While such an encoding method is commonly used, for a small object thebar code occupies a significant portion of the object and it detractsfrom the esthetics of the product. Also, a black and white bar code issusceptible to forgery once the code is deciphered or if the bar codeadheres to a known standard.

In response to the first of these shortcomings, bar codes that areessentially non-visible and which can be read with infrared radiationhave been developed. To implement the non-visible bar codes, thematerial deposited on the object or receiving medium must be invisibleunder radiation in the visible range but detectable under infraredradiation. Proposed materials included organic dyes such as cyaninebased dyes and naphthoquinone dyes (U.S. Pat. No. 5,911,921) andinorganic materials such as Ytterbium phosphate (U.S. Pat. No.5,911,921). The organic dyes are not stable in harsh environments andhave some selective absorption in the visible range. The inorganicmaterials can be expensive to manufacture although the Ytterbiumphosphate powder disclosed and claimed in U.S. Pat. No. 5,911,921 couldrepresent a lower cost solution.

However, any bar code implemented with a material having high absorptionin near infrared does not pose a solution to the problem of preventingforgeries since the use of an infrared scope or viewer will make the barcode detectable and the bar code could be counterfeited using infraredabsorbers.

One solution, disclosed in U.S. Pat. No. 5,760,384, is to use a materialthat absorbs infrared radiation within a narrow band of wavelengths. Oneembodiment disclosed in U.S. Pat. No. 5,760,384 is a phthalocyaninewhich could exhibit instability in harsh environments.

An alternate approach to solving the problem of preventing forgeries isto detect the forgeries. U.S. Pat. No. 5,760,384 also discloses anapparatus and method for judging whether a bar code, comprising amaterial that absorbs infrared radiation within a narrow band ofwavelengths, is real or a forgery. The disclosed method comprisesdetecting the reflectance at the peak absorption wavelength and atanother neighboring wavelength away from the peak absorption wavelength.

U.S. Pat. No. 5,336,252 proposes another approach to preventingforgeries of infrared bar codes. In U.S. Pat. No. 5,336,252 the infraredbar code is covered with a layer of an ink that has high absorption inthe visible and is transparent to the infrared. One embodiment is an inkprepared by mixing and dispersing a white pigment, such as titaniumoxide or zinc oxide, with an extender such as calcium carbonate. Whilethis approach will make it more difficult to forge the bar code, oncethe presence of the barcode is detected, it is not forgery proof.

All the above inventions relate to encoding the information in a markingcreated by depositing onto a medium a material that is capable ofstrongly absorbing radiation over a range of infrared wavelengths andsubstantially non-absorbing in the visible wavelengths. The necessity ofstrong absorption is derived from the decoding requirement that thereflectance, in the infrared range of wavelengths, from the infraredabsorbing material is significantly different than that from the medium.This requirement will ensure that the electrical signal from thedetector that receives the reflected infrared radiation is sufficientlydistinct from the background noise so that it can be reliably decoded.

The large group of materials that are capable of mildly absorbingradiation over a range of infrared wavelengths and substantiallynon-absorbing in the visible wavelengths, such as most syntheticpolymers (for example, polystyrene, Polyethylene terethalate), do notfind application in the encoding of information as markings on a mediumsince the resulting electrical signal would not be sufficiently distinctfrom the background noise to be reliably decoded if read with theoptical reader previously described. These large group of materialsincludes many low cost materials that would be attractive candidatematerials if the information encoded in marks could be decoded.

The object of the present invention is to present methods for the use oflower cost, stable materials to encode information onto or on a basemedium, and method and apparatus for reading the encoded informationwhile retaining robustness to forgeries.

SUMMARY OF THE INVENTION

Information is encoded in markings on a base medium by depositing orintertexturing on the base medium a material where the surfacedimensions, thickness and presence of the material contain the encodedinformation. In this invention, the material is capable of mildlyabsorbing radiation over a wide range of infrared wavelengths andsubstantially non-absorbing in the visible wavelengths. The encodingutilizes a lower cost, more stable material than a material that iscapable of highly absorbing over a range of infrared wavelengths andsubstantially non-absorbing in the visible wavelengths. Inventivemethods are then needed to ensure that the electrical signal from thedetector that receives the infrared radiation reflected or transmittedfrom the medium and material disposed thereon is sufficiently distinctfrom the background noise so that it can be reliably decoded. Twodifferent approaches for embodiments of such inventive methods aredisclosed. In all embodiments disclosed, at least one of a plurality ofsources of radiation is scanned over the medium and material disposedthereon, wherein said sources emit infrared radiation over a range offrequencies. Also in all embodiments disclosed, a portion of theradiation is reflected or transmitted from the medium and materialdisposed thereon, said reflected portion having a variable intensityover the scan. The reflected or transmitted portion is collected at adetector.

For one embodiment, the electrical signal from each of the at least oneof a plurality of detectors is sampled at each of a plurality of spacedapart locations on the medium and material disposed thereon, theposition of such locations being along the scan. At each of theplurality of spaced apart locations on the medium and material disposedthereon, the reflectance or transmittance at a plurality of selectwavelengths, selected from the range of wavelengths emitted by the atleast one of a plurality of sources, is determined. Knowledge of thereflectance or transmittance at a plurality of select wavelengths, whichis the same as knowledge of the reflectance or transmittance spectrum ata plurality of select wavelengths, allows the use of spectrumidentification methods, such as neural networks or principal componentanalysis, to identify the thickness and presence of the material at eachof the plurality of spaced apart locations. Once the presence of thematerial at each of the plurality of spaced apart locations has beenidentified, the surface dimensions of the material can be deduced fromthe presence of the material at each of the plurality of spaced apartlocations. From the thickness and surface dimension of the material, theencoded information can be decoded.

In another embodiment, the material disposed on the medium exhibits atleast one relative absorption maximum in the infrared in addition tobeing capable of mildly absorbing radiation over a wide range ofinfrared wavelengths and substantially non-absorbing in the visiblewavelengths. (The relative absorption maximum could be narrow or broad).The range of detectable wavelengths in the radiation reflected ortransmitted from the medium and material deposited thereon isrestricted. In one version of this embodiment, the restriction isselected to include one relative absorption maximum in the infrared. Therestricted portion of the reflected or transmitted radiation iscollected at a detector where it is generates an electrical signal, saidsignal having a variable intensity over the scan. The thickness andpresence of the material are identified from the electrical signal. Fromthe thickness and surface dimension of the material, the encodedinformation can be decoded.

In another version of the second embodiment, an oscillation is inducedin the intensity of the restricted portion of the reflected ortransmitted radiation collected at a detector, said oscillation havingcharacteristics that vary along the scan. Inducing the oscillation inthe collected radiation provides the opportunity to use known methods toincrease the signal-to-noise ratio of the electrical signalcorresponding to the collected radiation. After this step, the method isthe same as the preceding version of the embodiment.

Apparatus providing means to perform the steps of each of the abovedescribed method are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of this invention are set forth in the appendedclaims. However, the invention will be best understood from thefollowing detailed description when read in connection with theaccompanying drawings wherein:

FIGS. 1A-1D are respectively a graphical representation of the crosssection of a three dimensional bar code, the cross sectional view of atwo dimensional bar code, the top view of an area marking, the top viewof an intertextured structure;

FIGS. 2A-2D depict the spectra of several materials typical of thoseused in this invention;

FIG. 3 depicts the spectrum of a material having a high narrowabsorption peak in the infrared;

FIG. 4 depicts the spectrum of a material that has high absorption inthe infrared;

FIG. 5 depicts a typical spectrum of a planarizing material that issubstantially differentiated in the infrared wavelengths from thematerials used in this invention;

FIG. 6 is a graphical schematic representation of an apparatus used forretrieving the information encoded in a marking by detecting thespectrum in the infrared from the marking on a medium;

FIG. 7 depicts a spectrum and its second derivative;

FIG. 8 is a graphical schematic representation of an apparatus used forretrieving the information encoded in a marking where the detection isaided by conditioning the signal by restricting the range of detectablewavelengths;

FIGS. 9A-9B depict wavelengths of interest restricted to either oneregion in the information carrying material exhibiting a relativeabsorption maximum in the infrared or to two regions, one at the peak ofa relative absorption maximum and another at the trough of a relativeabsorption maximum;

FIG. 10 is a graphical schematic representation of an apparatus used forretrieving the information encoded in a marking where the detection isaided by conditioning the signal by restricting the range of detectablewavelengths and also inducing an oscillation in the intensity of thecollected radiation;

FIG. 11 depicts the manner in which the periodic tilting or oscillatingof an optical filter causes its center wavelength to oscillate;

FIG. 12 depicts the manner in which an optical filter that is tiltedperiodically sweeps over the relative absorption maximum.

DETAILED DESCRIPTION

In many applications where identifying product information forpurchasing and inventory control or personal or security information inID cards is provided, the information is encoded in markings on a basemedium by depositing or intertexturing on the base medium a materialwhere the surface dimensions and thickness of the material contain theencoded information. Examples of such markings are bar codes, areamarkings or three dimensional bar codes. FIG. 1A depicts the crosssection of a three dimensional bar code, where 1, 2, 3 are differentmaterials used for encoding information and 4 is a base medium. FIG. 1Bdepicts the cross sectional view of a two dimensional bar code; FIG. 1Cdepicts a top view of an area marking. FIG. 1D depicts the top view ofan intertextured structure.

The materials used in encoding information in the present invention,such as in the examples depicted in FIGS. 1A-1D, are capable of mildlyabsorbing radiation over a wide range of infrared wavelengths, mayexhibit at least one relative absorption maximum in the infrared, andare substantially non-absorbing in the visible wavelengths. Thedifference between a mildly absorbing material that may exhibit at leastone relative absorption maximum in the infrared and a highly absorbingmaterial is based on the physical cause of the absorption. Highlyabsorbing materials derive their absorption coefficient from electronicresonance while mildly absorbing materials derive their absorptioncoefficient from vibrational resonance and overtones. This definitionbecomes apparent upon comparison of the infrared absorption spectra ofthe materials. The spectra of materials typical of those used in thisinvention are shown in FIGS. 2A-2D. In contrast to those materials usedin the present invention, FIG. 3 depicts the spectrum a material havinga high narrow absorption peak in the infrared and FIG. 4 depicts thespectrum of a material that has high absorption in the infrared.

The materials used in the present invention include but are not limitedto transparent polymers, for example poly(isobutylmethacrylate).Synthetic polymers are excellent candidate materials since the overtonesfrom the vibrational motion from the hydrogen stretch normal mode ofC—H, N—H and O—H lead to absorption in the near infrared (NIR).Categories of materials and examples of such include but are not limitedto those given in Table 1. These materials are capable of mildlyabsorbing radiation over a wide range of infrared wavelengths and aresubstantially non-absorbing in the visible wavelengths. Polyethyleneterethalate (PET) and polystyrene are examples of materials that arecapable of mildly absorbing radiation over a wide range of infraredwavelengths, are substantially non-absorbing in the visible wavelengthsand exhibit at least one relative absorption maximum in the infrared.

TABLE 1 Materials for NIR Markings General Category Sub-CategoryExamples Main Chain Acyclic Poly(dienes) Polybutadiene Carbon PolymersPolyisoprene Poly(alkenes) Poly(butene-1) Polyethylene Poly(propylene)Poly(acrylics) Polyacrylic acid Poly(butylacrylate) Poly(acrylamides)Poly(N-butylacrylamide) Poly(methacrylics) Poly(butylmethacrylate)Poly(methylmethacrylate) Poly(methacrylamides) Poly(N-tert-butylmethacrylamide) Poly(vinyl ethers) Poly(vinylmethyether) Poly(vinylthioethers) Poly(methylthioethylene) Poly(vinyl alcohol) & Poly(vinylalcohol) Poly(vinyl ketones) Poly(benzoylethylene) Poly(vinyl halides) &Poly(vinyl chloride) Poly(vinyl nitriles) Poly(acrylonitrile)Poly(vinylesters) Poly(benzoyloxyethylene) Poly(styrenes)Poly(4-butylstyrene) Poly(α-methylstyrene) Poly(styrene) OthersPoly(vinyl carbazole) Poly(phenethylethylene) Main-ChainPoly(phenylenes) Poly(2,5-dimethyl-1,4- Carbocyclic phenyleneethylene)Polymers Main-Chain Poly(oxides) Poly(oxyethylene) Acyclic HeteroatomPoly(oxypropylene) Polymers Poly(phenylene oxide) Poly(carbonates)Polycarbonate of bisphenol A Poly(esters) Poly(ethylene terephthalate)Poly(anhydrides) Poly(oxyisophthaloyl) Poly(urethanes)Poly(oxycarbonylimino- decamethyleneimino- carbonyloxyhexa-decamethylene) Main-Chain Poly(siloxanes) Poly(dimethylsiloxane)O-Heteroatom Polymers Main-Chain Polysulfides Poly(thio-1,3,- —C—(S)—C—cyclohexylene) and —C—S—N— Poly(sulfones) Poly(oxy-1,4- Polymersphenylenesulfonyl-1, 4-phenylene) Main-Chain Poly(amides) Nylon 6—C—N—C— Nylon 6,6 Polymers Poly(benzyl glutamate) Poly(imines)Poly(benzoyliminoethylene) Poly(ureas) Poly(ureylenemethylene-1,4-phenylenemethylene ureylenedecamethylene) Other InorganicPoly(phosphazenes) Polymers Poly(silanes) Poly(germanes) Poly(stannates)Poly(titanates) Main-Chain Poly(benzoxazoles) HeterocyclicPoly(quinoxalines) Polymers Poly(benzimidazoles) Poly(oxindoles)Poly(triazines) Poly(pyridines) Poly(piperidines) Poly(triazoles)Poly(pyrazoles) Poly(pyrrolidines) Poly(phthalides) Poly(fluoresceins)Poly(dibenzofurans) Poly(anhydrides) Carbohydrates Amylose triacetateCellulose Cellulose triacetate Ethyl Cellulose Xanthan Dextran NaturalPolymers Alkyd & fossil resins Amber, kauri, congo, damar, ester gumNatural rubber Polyisoprenes Animal proteins Wool, silk, gelatin,collagen, Regenerated proteins Lanital, ardil, aralac, vicara, rayon etcVegetable cellulosic Cotton, kapok, abaca, fibers agave, falx, hemp,jute etc Enzymes Lysozome, trypsin, chymotrypsin Waxes Carnuba wax,paraffin wax Complex Starch, cellulose, Carbohydrates glycogen,carrageenan, agar, xylan, chitin, amylose

Table 1 lists a large number of polymers that are primarilyhomopolymers. The materials for this invention can include physicalmixtures or blends of many of these polymers. They can also includecopolymers of many of the monomers used to synthesize the polymerslisted in Table 1. The copolymers can be prepared from the monomers, bygrafting reactions, block and other typical coplymerization and polymerreaction pathways well known in the art (for the polymer reactions seeR. W. Lenz, Organic Chemistry of Synthetic High Polymers,ISBN470-52630-0.). The polymer layer can also include typical plasticadditives. Examples of additives are given below in Table 2.

TABLE 2 Additive Materials for NIR Markings Additives for NIR MarkingsGeneral Category Sub-Category Examples Processing additives Processingstabilizers Irganox 1010, Ionox 330, Polygard Lubricants Stearic acidFlow promoters acrylates Thixotropic agents Silica flour, bentoniteclays Mechanical property Plasticizers Di-octyl phthalate, modifiersadipic acid polyesters Reinforcing fillers Glass, silicon carbide,graphite Cost reducing Particulate fillers additives Diluents orextenders Surface property Anti-static agents Quarternary ammoniummodifiers salts, polyhydric alcohols and derivatives Slip additivesWaxes Anti-wear additives Anti-block additives Adhesion promotersAnti-ageing additives Anti-oxidants Irganox 1010, Ionox 330, Polygardu.v. stabilizers Fungicides Others Blowing agents Flame retardants

While these materials can be used for encoding information since theyare capable of mildly absorbing radiation over a wide range of infraredwavelengths, it is possible to attempt to decode an encoded marking ofthis invention by its topography. This concern could be answered bydepositing over the marking a planarizing layer of material. Theplanarizing material should be substantially differentiated in theinfrared range of wavelengths from the first material (the materialcapable of mildly absorbing in the infrared). Examples of suchplanarizing materials are most colored inks and combination of thoseinks that would render a visible black color. A typical spectrum of aplanarizing material that is substantially differentiated in theinfrared range of wavelengths from the at least one of a plurality offirst materials, black ink from a commercial marker (PaperMate W10permanent marker), is shown in FIG. 5. Another example of a planarizinglayer is an ink prepared by mixing and dispersing a white pigment, suchas titanium oxide or zinc oxide, with an extender such as calciumcarbonate, as previously described in the background section of thisapplication.

In cases where secure or not visible information is desired in additionto the visible markings, the present invention provides for depositingat least one layer of material that: is capable of mildly absorbingradiation over a wide range of infrared wavelengths, may exhibit atleast one relative absorption maximum in the infrared, and issubstantially non-absorbing in the visible wavelengths, over or under orbeside a layer of material that is capable of absorbing in the visibleand substantially differentiated in the infrared from the firstmaterial. If the infrared absorbing material or plurality of materialsis located underneath the material that absorbs in the visible, aplanarizing layer could be necessary. Such a planarizing layer would bechosen to provide contrast while reading the visible markings. Theplanarizing material should be substantially differentiated in theinfrared range of wavelengths from the first materials (the materialcapable of mildly absorbing in the infrared).

As should be apparent to those skilled in the art, the present inventioncan be combined with other materials in order to encode additionalinformation besides the visible marking.

If, in order to enhance detectability or to encode other information, itis desired to provide information encoded with materials that arestrongly absorbing in the infrared, the present invention provides fordepositing a layer of material that: is capable of mildly absorbingradiation over a wide range of infrared wavelengths, may exhibit atleast one relative absorption maximum in the infrared, and issubstantially non-absorbing in the visible wavelengths, over or under orbeside a layer of material that is capable of strongly absorbing in theinfrared and substantially non-absorbing in the visible.

Means of depositing the information carrying material, in the case of asingle layer of information carrying material, or depositing each of theplurality of materials, in the case of a multi-layer marking, onto thesurface of the medium include but are not limited to ink jet deposition,thermal transfer, vacuum deposition, pressure applied transfers (such asrub-on material), gravure printing, offset printing and screen printing.As it is clear to those schooled in the art, the deposition processincludes providing sources of the materials.

In one embodiment of the present invention, PVC or PVA is coated onto adonor ribbon. The material is subsequently transferred to the basemedium by means of a thermal transfer printer.

In another embodiment of the present invention, a clear ink isformulated with the preferred material (as described in this invention)as the binder. The material is then printed onto the base medium usingan ink jet printer.

In, still, another embodiment of the present invention, a wax isdeposited onto the base medium using a hot melt (thermal wax) printermechanism.

From the second of the above embodiments, it is apparent that the methodof this invention could be practiced by formulating a mixture ofpreferred materials (materials such as described in this invention) invarying proportional ratios, or in a combination with other materials,and encoding information by the presence or absence of the materials. Itis also possible to encode information in the surface dimensions andthickness. Also, the present invention can be combined with othermaterials in order to encode additional information besides the visiblemarking. One embodiment would be a material consisting of a visible dyein a binder material wherein the binder material can be varied andbelongs to the class of materials described above (capable of mildlyabsorbing radiation over a wide range of infrared wavelengths, possiblyexhibiting at least one relative absorption maximum in the infrared, andare substantially non-absorbing in the visible wavelengths.)

Means of intertexturing the information carrying material, in the caseof a single layer of information carrying material, or intertexturingeach of the plurality of materials, in the case of a multi-layermarking, with the base medium include but are not limited tointerweaving, embroidering and similar methods.

An embodiment of marking by intertexturing would consist of twodifferent fabrics (wool on cotton, or on synthetic material, forexample) interwoven so as to create a marking.

As can be seen from FIGS. 2, 3, and 4, the absorption of the materialsbeing used in this invention is significantly smaller than thosematerials that are capable of strongly absorbing radiation over a rangeof infrared wavelengths and substantially non-absorbing in the visiblewavelengths. Thus, in order to decode the information, means ofincreasing detectability are required. The means of increasingdetectability disclosed and claimed in this invention can be categorizedas either means of recognizing the presence of the material or means ofconditioning the signal so that known techniques of detection, such asbandwidth narrowing, can be applied successfully.

Detection by Recognition Techniques

The distinguishing characteristic of the materials being used forencoding information in this invention is a spectrum that exhibits mildabsorption of radiation over a wide range of infrared wavelengths andsubstantially no absorption in the visible wavelengths. Thus, acharacteristic to detect in order to identify the thickness and presenceof the material is the spectrum in the infrared.

FIG. 6 depicts a set-up for retrieving the information encoded inmarkings by detecting the spectrum in the infrared from a marking on amedium. Referring to FIG. 6, a source of radiation 61 that emitsinfrared radiation over a range of frequencies, such as a halogen lampor a tungsten lamp, a DC arc lamp, or a pulsed xenon arc lamp, isdirected towards scanning means 62, such as a mirror galvanometer,rotating mirrors, rotating mirror prisms, acousto-optical scanningsystems, and holographic scanning systems. (See O'Shea, Elements ofModern Optical Design, ISBN 0-471-07796-8, pp. 282-311). A plurality ofsources of radiation, such as an array of lasers emitting at selectwavelengths could also be used as sources of radiation. The scannedradiation illuminates a marking 63, such as a bar code, an area markingor a two dimensional bar code, and a portion of the radiation isreflected or transmitted from the marking, said reflected portion havinga variable intensity over the scan. Optical elements 64 direct thereflected portion of the radiation to an element 65, such as a gratingor a plurality of gratings or a prism, that disperses the radiation intoa plurality of select wavelengths in the infrared. The plurality ofselect wavelengths is taken from the range of wavelengths emitted by atleast one of the sources of radiation. Optical elements 66 focus thereflected dispersed radiation onto an array of detectors 67, where thereflected dispersed radiation is collected and converted into electricalsignals. The detector signals correspond to the plurality of selectwavelengths. One embodiment of the detectors would be lead sulfidedetectors. Other possible embodiments are pyroelectric detectors (usingdetector materials such as tryglycerine sulfide, strontium bariumniobate, polyvinylidene flouride or lithium tantalate), focal planearrays using materials sensitive in the near infrared (such arrays needan operating temperature lower than ambient) or InGaAs detectors.

A/D circuits 68 convert the analog electrical signals to signals definedat spaced apart points corresponding to the spaced apart locations onthe medium and material disposed thereon. The placement of saidlocations is along the scan. At each of the spaced apart points, theplurality of electrical signals determines the absorption (orequivalently reflectance or transmittance) spectrum at selectwavelengths 69. In a refinement of this embodiment, characteristics ofthe spectrum are calculated after obtaining the spectrum. One suchcharacteristic is the second derivative. Other possible characteristicsare the first derivative, third derivatives or higher derivatives. Suchcharacteristics are useful if the spectrum is not easily separated fromthe spectrum of the base medium. FIG. 7 depicts such a case andillustrates the advantage of using a second derivative of the spectrumto differentiate the material from the base medium.

While FIG. 6 depicts the reflected radiation case, it is obvious tothose schooled in the art that the transmitted radiation could be usedinstead. In the transmitted case, the transmitted portion of theradiation is dispersed into a plurality of select wavelengths in theinfrared. The transmitted dispersed radiation is collected and convertedinto electrical signals. The detector signals correspond to theplurality of select wavelengths. The identification and decoding stepsthen proceed in the same manner as for the reflected case.

An identifying circuit 70 receives the absorbency (or equivalentlyreflectance or transmittance), and its characteristics if any have beencalculated, at select wavelengths 69 at each of the spaced apartlocations and determines the thickness and presence of the material ateach of the spaced apart locations. One embodiment of such anidentifying circuit would contain a processor and a memory, where thememory contains spectrum information for the materials used. Theprocessor would be programmed to execute a spectrum identification andcalibration calculation where the spectrum is characterized by atechnique such as Multiple Linear Regression (MLR), Partial LeastSquares (PLS), Principal Component Analysis (PCA). (The PrincipalComponent Analysis calculation is detailed below since it is a spectraldata compression technique and is usually combined with othertechniques.) The spectrum, that is, the absorbency (or equivalentlyreflectance or transmittance) at select wavelengths, is compared to theknown spectrum of the material deposited on a medium or intertexturedwith a base medium and to the known spectrum of the base medium. Thiscomparison involves applying techniques of pattern recognition such asclustering, Euclidean distance analysis, Mahalanobis distance, neuralnetworks, fuzzy neural networks, fuzzy logic applied to clustering andothers. (For example, techniques for pattern recognition are describedin Jurs, Computer Software Applications in Chemistry, ISBN0-471-10587-2, Ch. 14, in Otto, Chemometrics, ISBN 3-527-29628-X, Ch.5and Ch. 8 and in Bezdek, Pal, Fuzzy Models for Pattern Recognition, ISBN0-7803-0422-5. For the Mahalanobis distance technique, see Workman,Springsteen, Applied Spectroscopy, ISBN 0-12-764070-3, Ch.5).

For identifying both the presence and thickness, both the identificationand calibration need to be performed (see Workman, Springsteen, AppliedSpectroscopy, ISBN 0-12-764070-3, Ch.4, 5). The presence of the materialis detected by spectral comparison. The above mentioned techniques ofpattern recognition are applied to the measured spectrum. It issometimes advantageous to decompose the spectrum into principalcomponents before performing the recognition. Multiple Linear Regression(MLR) can also be used to provide a representation of the spectrum.Calibration aims to relate the absorption to concentration, path lengthand absorbency. Calibration methods include MLR, PLS and PartialComponent Regression (PCR). Both PLS and PCR are related to thePrincipal Component Analysis.

The outputs of the identifying circuit are used to deduce the surfacedimensions of the material from presence of the material at each of theplurality of spaced apart locations.

(The deduced surface dimensions can be relative or absolute. An absolutemeasurement would require pre-existing calibration and might not benecessary for decoding. For example, the presence of the informationcarrying material for a given distance or a given area, the distancebeing represented by a time in the electrical signal, translates into avalue “1”; the surface dimension, in this case, is given by the timeover which the value is “1”.)

From the thickness and surface dimension of the material, the encodedinformation is decoded by the decoding circuit 71. In one embodiment,the thickness and surface dimensions are translated to a digital signal.For example, for a uniform thickness situation, the presence of theinformation carrying material for a given distance (or a given area),the distance being represented by a time in the electrical signal,translates into a binary number “1”. Deducing means would consist ofelectrical circuits that implement the representation. Extensions ofthis embodiment are well known to those skilled in the art.

Decoding means are well known to those skilled in the art. For a digitalsignal, the decoding can be implemented in a set of digital elementsthat converts one digital signal to another or it could be implementedusing a processor and memory (decoder circuits are described in inHorowitz, Hill, The Art of Electronics, ISBN 0-521-23151-5).

It should be apparent to those skilled in the art that the identifying,deducing and decoding means could be implemented in a digital computerwith the appropriate programmed software.

Principal Components Analysis

The mathematical basis of principal component analysis is familiar tothose skilled in the art (see, for example, Kshirsagar, MultivariateAnalysis, ISBN 0-8247-1386-9, Ch. 11). A brief summary is given as abasis for the required stored information in the identifying circuit.Principal Components Analysis (PCA) is a method of data reduction inwhich a spectrum is characterized by its uncorrelated components. Inorder to generate the stored information, previous calculations arerequired. The starting point is a description of the spectrum of theeach of the materials used to encode the information. Such descriptioncontains discrete spectrum values, S(λ_(i)), at discrete wavelengths,λ_(i), at frequent enough intervals to fully describe the spectrum. Thespectrum description can be considered a vector,

^(T)=[S(λ₁) S(λ₂). . . S(λ_(i)). . . S(λ_(n))]  (1)

where n is number of discrete wavelengths, and ^(T) denotes thetranspose of the vector .

The covariance matrix is the matrix defined by

ρ=^(T).  (2)

an n×n matrix.

The principal component vectors are the eigenvectors of the covariancematrix; (that is, the vector is the solution to

ρ=Λ,  (3)

where the Λs are the eigenvalues. There are n solutions to thisequation, _(i), i=1 to n. (Methods of solving the eigenvalue-eigenvectorproblem are well known in linear algebra and can be found in anyintroductory linear algebra text or a text on linear systems such as DeRusso, Roy, Close, State Variables for Engineers, Ch. 4).

The spectrum can now be represented as a sum of the eigenvectors,

Σd_(i) _(i)(λ), where the sum extends from 1 to n  (4)

The d_(i)s can be determined by inverting a matrix equation.

The eigenvectors can be rank ordered as to importance and typically lessthan n are needed to characterize the spectrum in the region ofinterest. For example, k eigenvectors are used to describe the spectrum,where k is less than n. In cases of interest k can be as small as 3. Theabsorption (or equivalently reflectance or transmittance) spectrum ofthe medium and material disposed thereon can be represented as

Σa_(i) _(i)(λ), where the sum extends from 1 to k.  (5)

The a_(i)s can be completely determined if the number of selectwavelengths at which the reflected portion of the radiation is collectedis equal to k. In spectroscopy, the representation of equation 5 iscalled the scores-factors representation, where the a_(i)s are calledscores and the _(i)(λ)s are called factors.

Comparing the measured spectrum (in its principal componentrepresentation) and the spectrum of each material reduces to comparingcoefficients. In order to account for variations, fluctuations andmeasurement noise, pattern recognition techniques (such as clustering,Euclidean distance analysis, neural networks, fuzzy logic processing)can be applied in identifying the presence of the material at each ofthe plurality of spaced apart locations.

Detection by Conditioning the Signal

For materials that in addition to being capable of mildly absorbingradiation over a wide range of infrared wavelengths and substantiallynon-absorbing in the visible wavelengths also exhibit at least onerelative absorption maximum in the infrared, the electrical signalresulting from the collected reflected radiation can be conditioned inorder to increase detection. Referring now to FIG. 8, a source ofradiation 81 is scanned, using scanning means 82, over a marking 83,consisting of material deposited or intertextured onto a base medium.The source of radiation 81 could be a broad source of radiation thatemits infrared radiation over a range of frequencies, such as a halogenlamp or a tungsten lamp or a laser that emits at a desired wavelength.The reflected portion of the radiation is collected at the detector ordetectors 85. Examples of infrared detectors include but are not limitedto lead sulfide detectors and pyroelectric detectors (using detectormaterials such as tryglycerine sulfide, strontium barium niobate,polyvinylidene fluoride or lithium tantalate).

While FIG. 8 depicts the reflected radiation case, it is obvious tothose schooled in the art that the transmitted radiation could be usedinstead. In the transmitted case, the transmitted portion of theradiation is collected at the detector or detectors. After the entranceto the detector or detectors, the two cases proceed identically.

The range of detectable wavelengths in the reflected (or transmitted)portion of the radiation that reaches the detector 85 is restricted tolocations in the spectrum exhibiting at least one relative absorptionmaximum in the infrared by restricting means 84. One embodiment of therestricting means would be at least one optical filter. The filter canbe either at the source, or at the detector, or both, or in between, asmay be smallest and most convenient. The range also could be restrictedat the source by utilizing at least one source that emits only in arange of frequencies (for example, a laser that emits at the desiredwavelength). The wavelengths of interest are restricted to a region inthe information carrying material, said region exhibiting a relativeabsorption maximum in the infrared (refer to FIG. 9A). Reflectedradiation from the markings can then be collected optically anddelivered onto a detector of any reasonable size. The detector convertsthe collected reflected radiation to an electrical signal 86. Theelectrical signal is input to a deducing circuit 87 that produces asignal representing the thickness and presence of the deposited materialover the scan. Deducing means are known to those schooled in theelectrical arts and include but are not limited to comparing to athreshold, DC removal and zero crossing detection (comparator circuits,which are sometimes used for determining presence, are discussed inHorowitz, Hill, The Art of Electronics, ISBN 0-521-23151-5;differentiation and zero crossing detection are also used for edgedetection, another technique for presence detection; other techniquesshould be apparent to those schooled in the instrumentation arts).Finally, a decoding circuit 88 decodes the information encoded in thethickness and surface dimension of the deposited material. Byrestricting the wavelengths of interest to the region in informationcarrying material exhibiting a relative absorption maximum in theinfrared, a higher contrast is obtained between the signal from theinformation carrying material and the signal from the base medium. Thishigher contrast results in improved detectability of the presence of thedeposited material.

Again, in one embodiment, the thickness and surface dimensions aretranslated to a digital signal. For example, for a uniform thicknesssituation, the presence of the information carrying material for a givendistance (or a given area), the distance being represented by a time inthe electrical signal, translates into a binary number “1”. Deducingmeans would consist of electrical circuits that implement therepresentation. Extensions of this embodiment are well known to thoseskilled in the art.

Decoding means are well known to those skilled in the art. For a digitalsignal, the decoding can be implemented in a set of digital elementsthat converts one digital signal to another or it could be implementedusing a processor and memory (decoder circuits are described in inHorowitz, Hill, The Art of Electronics, ISBN 0-521-23151-5).

It should be apparent to those skilled in the art that the identifying,deducing and decoding means could be implemented in a digital computerwith the appropriate programmed software.

One possible embodiment of the deducing means can further include usingtwo detectors (or a pyroelectric detector where the opposing sensitiveareas of the pyroelectric detector may be superimposed spatially). Themarkings are detected by a moving small image of one of the detectors(or one of the pyroelectric elements), but are at the same time viewedby a diffused or defocused (lower resolution) image of the otherdetector (or opposing pyroelectric element), to enhance detection of theedges (by increasing resolution) of the markings. (In a photographicanalogy the same technique is called unsharp masking.) In analogy tounsharp masking, the electrical signal from the diffused image issubtracted from the electrical signal corresponding to the image of themarkings to produce a difference (high frequency) electrical signal. Anincreased resolution electrical signal is obtained by adding a fractionof the difference (high frequency) electrical signal to the electricalsignal corresponding to the image of the markings. Since edges areenhanced, if the increased resolution electrical signal is utilized todeduce the thickness and presence of the deposited material over thescan, the probability of detection of the presence of the depositedmaterial should improve.

In the case where a laser or several lasers that emit at the desiredwavelengths are used, more power can be used safely in a broadened beamto illuminate the area to be scanned. If the radiation passes firstthrough two diffusers, the eye will never be exposed to a concentratedsource. In this configuration the scanning would be accomplished byforming a moving small image of the detector area onto the plane of themarkings.

In the case where a broad source of radiation that emits radiation overa range of frequencies, such as a halogen lamp or a tungsten lamp, isused to illuminate the area containing the markings, a suitablesmall-area interference filter can be used to isolate the wavelengthband to be sensed. The filter can be either at the source, or at thedetector, or both, or in between, as may be smallest and mostconvenient. A tiny image of the detector would be used to scan themarkings.

It is possible to further improve the probability of detection of thepresence of the deposited material by processing the signal with a“matched” electrical filter. (In the case where it is only necessary todetermine whether or not a signal is present, an electrical filter thatmaximizes the ratio of output signal power to output noise power will beoptimum in the mean square error sense. Knowing the signal propertiesand the noise properties, it is possible to derive such a filter.“Matched” filters are known in communication systems theory. See, forexample, Cooper, McGillem, Modern Communications and Spread Spectrum,ISBN 0-07-012951-7, pp. 82-91). For a single pass scanning systemmodified to read absorption markings or strips at wavelength λo, the IRabsorption signal pulse will, typically, have a Gaussian shape with apulse duration dictated by the velocity of the probe beam passing overthe IR absorption marking, the spatial extent of the IR detecting probeon the IR absorption marking, and the size of the IR absorption markingalong the scan direction. To achieve maximum S/N ratio (detection) forsuch a signal, a “matched” filter, preferably placed just after theoutput of the detector/preamplifier, should be employed. The “matched”filtering would probably be done in the digital domain by first A/Dconverting the detector/preamplifier output at a frequency sufficientlyhigher than the reciprocal of the duration of the IR absorption signalpulse, and a high speed DSP to do the “matched” filtering in real time.The shape, duration, and potential repetition rate of the IR absorptionsignal pulse will govern the design of the “matched” filter. Toaccommodate a range of distinctly different shaped IR absorption signalsdue to different size markings, different scan angles over the markings,different distances of the marking from the scanner, different IRabsorption materials with different signatures, etc., a bank of two ormore matched filters in parallel could be employed. The outputs from thefilters would be compared to determine both the existence of an IRabsorption marking and (depending on which filter had the strongestsignal) the type or character of the IR absorption marking. The bank of“matched” filters could possibly be accomplished using just one DSP andmicroprocessor with cache memory in real time. Also, the relativeposition of the marking(s) can be determined by the one to onerelationships between the time of detection and position in the scan. Ifrepeat scans are permissible, then the S/N ratio of the “matched” filterscheme would improve by the square root of the number of repeat scanstaken.

It is also possible to further improve the probability of detection ofthe presence of the deposited material by dividing the restricted rangeof detectable wavelengths in the reflected portion of the radiation intoa plurality of regions. This could be accomplished by using a pluralityof filters and detectors. In one embodiment, two regions are selectedwhere one of the regions is at the peak of one relative absorptionmaximum and the other region is at the trough of the same relativeabsorption maximum (refer to FIG. 9B). This could be implemented using apyroelectric detector where the opposing sensitive areas of thepyroelectric detector may be superimposed spatially but filtered forslightly different wavelength response by narrow-band interferencefilters. (Pyroelectric detectors usually have of a pair of sensitiveresponse areas near each other, but giving opposite electricalpolarity.) It could also be implemented using two separate detectors.The reflected radiation in each region of the wavelength range willproduce two electrical signals. Each electrical signal is produced byone detector or one sensitive area of a single detector. One electricalsignal is produced by collecting the radiation from the region at thepeak of one relative absorption maximum and the other electrical signalis produced by collecting the radiation from the region at the trough ofthe same relative absorption maximum. The two electrical signals aresubtracted to produce a difference signal which will have bettersignal-to-noise since the common mode noise will be eliminated. Again, a“matched” electrical filter can be used to further improve theprobability of detection of the presence of the deposited material.

In another embodiment, three regions are selected where one of theregions is at the peak of one relative absorption maximum and the otherregions are at either trough of the same relative absorption maximum.This embodiment could be implemented using three separate detectors. Thereflected radiation in each region of the wavelength range will producethree electrical signals. Each detector produces one electrical signal.One electrical signal is produced by collecting the radiation from theregion at the peak of one relative absorption maximum and the other twoelectrical signals are produced by collecting the radiation from theregion at either trough of the same relative absorption maximum. Theelectrical signals from the region at either trough of the same relativeabsorption maximum are averaged and the resulting average signal issubtracted from the signal produced by collecting the radiation from theregion at the peak of one relative absorption maximum. The differencesignal will have better signal-to-noise since the common mode noise willbe eliminated. Again, a “matched” electrical filter can be used tofurther improve the probability of detection of the presence of thedeposited material.

As should be apparent to one skilled in the art, if the range isrestricted at the source by utilizing at least one source that emitsonly in a range of wavelengths (for example, lasers that emit at thedesired wavelengths) and the range is divided into a plurality ofregions, the collecting means should include means of directing eachsub-range (that is, region) to a separate detecting surface.

Another method for further improving the probability of detection of thepresence of the deposited material comprises, besides scanning theradiation over the markings and base medium, restricting the range ofdetectable wavelengths and collecting the restricted portion ofreflected radiation, the additional step of inducing an oscillation inthe intensity of the collected radiation, said oscillation havingcharacteristics that vary along the scan.

Referring to FIG. 10, a source of radiation 91 is scanned, usingscanning means 92, over a marking 93, consisting of material depositedor intertextured onto a base medium. The source of radiation 91 could bea broad source of radiation that emits infrared radiation over a rangeof frequencies, such as a halogen lamp or a tungsten lamp.

While FIG. 10 depicts the reflected radiation case, it is obvious tothose schooled in the art that the transmitted radiation could be usedinstead. In the transmitted case, the transmitted portion of theradiation is collected at the detector or detectors. After the entranceto the detector or detectors, the two cases proceed identically.

The reflected portion of the radiation is collected at the detector 96,where the detector could be any infrared detector, for example, any ofthe previously mentioned detectors. The range of detectable wavelengthsin the reflected portion of the radiation that reaches the detector 96is restricted to locations in the spectrum exhibiting at least onerelative absorption maximum in the infrared by restricting means 94. Oneembodiment of the restricting means would be an optical filter. Thefilter can be either at the source, or at the detector, or both, or inbetween, as may be smallest and most convenient. Oscillation inducingmeans 95 induce an oscillation in the intensity of the restrictedportion of the reflected radiation collected at a detector, saidoscillation having characteristics that vary along the scan. Oneembodiment of the oscillation inducing means is means to pivot thefilter about its center, oscillating it with a small amplitude. Suchmeans could be similar to the scanning means previously described. Thisperiodic tilting of the filter causes its center wavelength to oscillateas shown in FIG. 11.

Another embodiment of the oscillation inducing means utilizes anarrow-band multilayer interference filter with a peak wavelength thatvaries with position across the plate (the plane of the filter). Thefilter design involves one or more layers with a thickness changinguniformly with position. Spinning such a plate about an axisperpendicular to its surface and suitably placing it on the gradient ofthickness of the variable thickness layers, the peak wavelengthtransmitted just to one side of that axis will vary sinusoidally withrotation between two limit values, the range dependent on thedecentration. The plate could be supported with a suitable number ofsimple springs so that it could be made to oscillate in its plane bysimple resonance, driven by a changing magnetic field or shaken by asmall motor with an eccentric weight. Again the peak wavelength willvary sinusoidally, between limits defined by the gradient and theamplitude of resonant travel, in a manner as shown in FIG. 11.

The filter then sweeps over the relative absorption maximum as shown inFIG. 12. This sweeping over the relative maximum, if properly arranged,moves the center wavelength from the peak of the relative absorption toeither one of the troughs, thereby inducing an oscillation in intensityof the collected radiation. The electrical signal 97 produced by thedetector 96 in response to the collected radiation will be modulated bythe oscillation. The modulating signal has the effect of shifting theinformation content from low frequencies to the modulating frequency.The identifying means 98 would include a band limited electrical filter,which would select the electrical signal in the vicinity of themodulating frequency. By judicious choice of the modulating frequency,the identifying will occur in the presence of reduced detector noise(See, for example, FIGS. 4-8 in Dereniak, Crowe, Optical RadiationDetectors, ISBN 0-471-89797-3) which would result in improving theprobability of detection of the presence of the deposited material.Again, a “matched” electrical filter can be used to further improve theprobability of detection of the presence of the deposited material.

Other embodiments of the invention, including combinations, additions,variations and other modifications of the disclosed embodiments will beobvious to those skilled in the art and are within the scope of thefollowing claims.

What is claimed is:
 1. A method of encoding information onto a mediumcomprising the steps of: providing sources for at least one of aplurality of materials, said materials mildly absorbing radiation over arange of infrared wavelengths and substantially non-absorbing in visiblewavelengths; and depositing said at least one of a plurality ofmaterials onto the surface of a medium in varying surface dimensions andthickness of deposition such that the information is encoded in saidsurface dimensions and thickness.
 2. The method of claim 1 furthercomprising the steps of: providing a source for a second material, saidsecond material strongly absorbing radiation over a range of infraredwavelengths and substantially non-absorbing in the visible wavelengths;and depositing said second material onto the surface of the medium invarying surface dimensions and thickness of deposition such thatinformation is encoded in said surface dimensions and thickness.
 3. Themethod of claim 1 further comprising the steps of: providing a source ofa second material, said second material absorbing radiation in thevisible range of wavelengths and substantially differentiated in theinfrared range of wavelengths from said at least one of a plurality offirst materials; and depositing said second material onto the surface ofthe medium in varying surface dimensions and thickness of depositionsuch that information is encoded in said surface dimensions andthickness.
 4. A method of encoding information on a medium, said mediumbeing comprised of a base material and at least one information carryingmaterial, comprising the steps of: providing sources for at least one ofa plurality of materials, said materials mildly absorbing radiation overa range of infrared wavelengths and substantially non-absorbing invisible wavelengths; and intertexturing said at least one of a pluralityof information carrying materials with the base medium wherein eachintertextured information carrying material has varying surfacedimensions and thickness such that the information is encoded in saidsurface dimensions and thickness.
 5. The method of claim 1 or claim 4further comprising the steps of: providing a source of a planarizingmaterial, said planarizing material being substantially differentiatedin the infrared range of wavelengths from said at least one of aplurality of materials; and depositing said planarizing material ontothe surface of the medium and information carrying material disposedthereon.
 6. The method of claim 1 or claim 4 wherein the said materialsalso exhibit at least one relative absorption maximum in the infrared.7. The method of claim 4 further comprising the steps of: providing asource for a second material, said second material strongly absorbingradiation over a range of infrared wavelengths and substantiallynon-absorbing in the visible wavelengths; and intertexturing said secondmaterial with the base medium wherein the intertextured informationcarrying second material has varying surface dimensions and thicknesssuch that the information is encoded in said surface dimensions andthickness.
 8. The method of claim 2 or claim 7 further comprising thesteps of: providing a source of a planarizing material, said planarizingmaterial being substantially differentiated in the infrared range ofwavelengths from said at least one of a plurality of first materials;and depositing said planarizing material onto the surface of the mediumand information carrying material disposed thereon.
 9. The method ofclaim 2 or claim 7 wherein at least one of said plurality of firstmaterials also exhibit at least one relative absorption maximum in theinfrared.
 10. The method of claim 4 further comprising the steps of:providing a source of a second material, said second material absorbingradiation in the visible range of wavelengths and substantiallydifferentiated in the infrared range of wavelengths from said at leastone of a plurality of first materials; and intertexturing said secondmaterial with the base medium wherein the intertextured informationcarrying second material has varying surface dimensions and thicknesssuch that the information is encoded in said surface dimensions andthickness.
 11. The method of claim 3 or 10 further comprising the stepsof: providing a source of a planarizing material, said planarizingmaterial being substantially differentiated in the infrared range ofwavelengths from said at least one of a plurality of first materials;and depositing said planarizing material onto the surface of the mediumand information carrying material disposed thereon.
 12. The method ofclaim 3 or claim 10 wherein at least one of said first plurality ofmaterials also exhibit at least one relative absorption maximum in theinfrared.
 13. A method of encoding information onto a medium comprisingthe steps of: providing sources for at least two of a plurality of firstmaterials, said materials mildly absorbing radiation over a range ofinfrared wavelengths and substantially non-absorbing in visiblewavelengths; providing a source of a second material, said secondmaterial being capable of absorbing radiation in the visible range ofwavelengths and substantially differentiated in the infrared range ofwavelengths from said at least two of a plurality of first materials;combining the second material with a quantity of the at least two of theplurality of first materials, wherein the at least two of the pluralityof first materials are in varying proportional ratios to constitute aquantity of first materials; and depositing said combination ofmaterials onto a surface of the medium in varying surface dimensions,proportional ratio of the at least two of the first materials andthickness of deposition such that the information is encoded in saidsurface dimensions, proportional ratio of the at least two of the firstmaterials and thickness.
 14. The method of claim 13 wherein the secondmaterial is a visible dye and the at least two of the plurality of firstmaterials are binders.
 15. The method of claim 13 or 14 furthercomprising the steps of: providing a source of a planarizing material,said planarizing material being substantially differentiated in theinfrared range of wavelengths from said at least two of a plurality offirst materials; and depositing said planarizing material onto thesurface of the medium and information carrying material disposedthereon.
 16. The method of claim 13 or claim 14 wherein at least one ofsaid at least two first materials also exhibits at least one relativeabsorption maximum in the infrared.